In the area of position encoding, and more specifically in the absolute encoder art, high resolution position encoders typically employ a multiplicity of separate tracks which are read directly to provide position words. Alternatively, there is used a combination of direct read track and an additional track. The additional track provides the least significant bits of the output position word. The latter approach is illustrated in U.S. Pat. No. 3,534,360 to Hafle. In such an approach the least significant bit of the position words from the direct read tracks is synchronized with the most significant bit of the resolved word using an XOR function.
The disadvantages of the above approaches include: (1) the greater number of direct read tracks, the higher the physical area required to support such tracks, and (2) the merging of resolved words with direct-read words is limited in the amount of variation which can be tolerated between the resolved words and the directly read words.
Other prior approaches have extended the merging concept disclosed in the above referenced patent to Hafle, and merged position words synthesized from different tracks of overlapping resolution. This is reflected in Model Mil-158 developed by the inventor of the present application and sold by BEI Motion Systems, Inc., the assignee of the present application.
More specifically, several tracks having overlapping resolution are used to synthesize position words of different but overlapping resolution. The tracks are used to generate triangular wave signals. The amplitude from each signal is converted into a binary position word using analog to digital converters. These position words are then merged together with the overlapping bits between position words being used to correct the least significant bit of the next coarser resolution position words. The relative resolution between tracks is selected so that an overlap of at least two bits between position words is provided.
In this manner, high resolution position words can be assembled by merging position information resolved from a plurality of different tracks of indica. The different tracks of indicia each provide position information having different, but overlapping, degrees of resolution so that when position information from each of the tracks are merged or synchronized, a position word of higher resolution is obtained.